The final day of the NFL Scouting Combine has arrived, and that means defensive backs are being tested in the 40-yard dash.

This measurement -- especially the initial 10-yard burst -- arguably is more important for cornerbacks than any other position on the field, adding gravity to Tuesday's impressive run by Mississippi State's Darius Slay.

Slay posted an official time of 4.36 seconds in his second 40 run to finish with the fastest time on the day. Slay came to the combine slated as a middle-round selection, but his speedy performance should help him move up boards with the 2013 NFL Draft less than two months away.

Alabama's Dee Milliner fared very well in his second run, recording an official time of 4.37 to finish second overall. Milliner is considered one of the top secondary prospects available in the draft.

Cal's Steve Williams, who initially lifted eyebrows with an unofficial 4.25, finished with a much slower official time of 4.42.

Other notes from Tuesday's session:

» One of the more disappointing runs of the session befell Johnthan Banks. The 6-foot-2, 185-pound Mississippi State corner recorded a 4.61, a sluggish mark that stands to damage his draft stock. Texas' Kenny Vaccaro also struggled, posting a lower-than-expected 4.63.

» NFL.com's Daniel Jeremiah listed Banks along with Florida State's Xavier Rhodes as two prospects who needed to flash solid times in order to secure their first-round status. You can hide a lack of speed at positions all over the field, but that's nearly impossible from your cornerbacks. While Banks floundered, Rhodes finished with a strong 4.43.

» Former LSU cornerback Tyrann Mathieu clocked in with a strong official time of 4.50. Mathieu is considered one of the draft's biggest unknown quantities, but it appears he has the speed to perform at the next level.

» NFL.com's Gil Brandt told Around The League that guys like Banks have to bring it in the cone and short shuttle to redeem themselves. Those drills show that these players can quickly change direction, a key to pass coverage. A bad 40 doesn't necessarily kill them if they exhibit strong cone or shuttle work.

» Florida free safety Matt Elam recorded a 4.54 in his first attempt. Not blazing speed, but not bad for an undersized safety.